Interfacial structure and properties of TiO2 phase junction studied by DFT calculations

Mixed-phase (i.e. phase junction) TiO₂ has been widely used in commercial photocatalysis applications, because of its excellent photocatalytic performance. However, the interfacial composition, micro-structure, and properties of mixed-phase TiO₂ are not fully understood so far. Consequently, the phase transition process TiO₂ from anatase to rutile is still not entirely clear; and there is no consensus on the mechanism of mixed-phase enhancing photocatalytic performance of TiO₂. In this article, 16 interface models for TiO₂ phase junctions were constructed by the low-index surfaces of TiO₂ with anatase and rutile phases. The interfacial energy, microstructure, electronic structure and properties have been systematically calculated and analyzed. Based on the interfacial formation energy and the configuration of [TiO₆] octahedron at A/R interface, the phase transition of TiO₂ is most likely to occur at the interface between anatase (110) plane and rutile (011) plane, owing to the favorable interface formation energy and the smallest deformation of [TiO₆] octahedrons. However, because the migration direction of photo-generated carriers is opposite to the direction of the built-in electric field at the A/R interface in A(110)/R(011) phase junction, the hetero-direction migration of photo-generated electron-hole pairs will be prevented. Thereby, the existence of A(110)/R(011) phase junction is unfavorable to improve the photocatalytic performance. On the other hand, when the calcination temperature rises, the favorable phase junctions (such as A(110)/R(001) phase junction) gradually occupies the growth advantage, which can improve the photocatalytic performance of TiO₂. This is a possible explanation for the excellent photocatalytic properties of Degussa P25 with mixed-phase.